Electroluminescent light amplifier



g 2, 1965 s. c. PEEK, JR 3,

ELECTROLUMINESCENT LIGHT AMPLIFIER Filed Jan. 31. 1955 IN VEN TOR. 5A NDFORD CHRISTOPHER PEEK, JR

FIG. 5 v W [g t/U ATTORNEY United States Patent M 3,264,479 ELECTROLUMINESCENT LIGHT AMPLIFIER Sandford C. Peek, Jr., Hamilton, Mass., assignor, by mesne assignments, to Sylvania Electric Products Inc.,

Wilmington, Del., a corporation of Delaware Filed Jan. 31, 1955, Ser. No. 485,131 2 Claims. (Cl. 250213) This invention relates to electroluminescent devices, that is to devices in which light is produced by electrical action in a solid material. The latter is generally in the form of small crystals or particles, and is often called a phosphor.

I have discovered that light amplification can be obtained with such devices, so that a small intensity of incident light can be used to control the emission of a higher intensity of light. For example, an image of low intensity can be focused on a screen according to my invention, and a corresponding image of much higher intensity obtained therefrom on the screen.

By my discovery, such a result can be achieved by placing a photoconductive screen over an electroluminescent screen, and exciting both screens in series between a pair of electrodes, at least one of the latter being of the transparent conductive type. The electroluminescent screen can be, for example, of copper-activated Zinc sulfide, and the photoconductive screen of selenium, cadmiurn sulfide, zinc sulfide or the like.

The photoconductive screen can be built up of a mosaic of separate particles to reduce the tendency to sideward or parallel flow of the current, which would make the image less sharp.

With a fixed voltage placed across the two screens in series, any change in conductivity produced in the photoconductive screen by the incident light will produce a change in the voltage across the electroluminescent screen and hence a change in its light output. The resistance of the photoconductive screen or layer in the absence of exciting light, should be high compared to the impedance of the electroluminescent screen or layer in order that changes in the resistivity produced by the exciting light will cause considerable variation in the voltage across the electroluminescent material. The voltage impressed across the layers in series should be high enough to insure sufficient brightness of the electroluminescent layer when incident light falls on the photoconductive layer, and the resistance of the photoconductive layer should be low enough to allow sufiicient voltage across the electroluminescent layer for a desired brightness at the minimum light intensity to which response is desired, and high enough to insure very little brightness when there is no incident light.

In addition to modulating a high intensity of light by a weaker intensity of light, my device can vary the intensity of the light at different points on the transverse area of the electroluminescent layer, thereby producing an image on the latter in response to a weaker image incident on the photoconductive layer.

It the light emitted by the electroluminescent layer is in the wavelength range to which the photoconductive material responds and is not screened from the photoconductive layer at least in part, it can feed back onto the photoconductive layer to maintain the light output even after the exciting radiation has been cut off, thus giving a light relay in which a small intensity of light will not only be amplified to a higher intensity but will also remain at the high intensity until the voltage is removed or reduced. Such a result is often desirable.

However, if a screen is provided, inherently or otherwise, between the electroluminescent and photoconductive layers, the light produced by the electroluminescent electrical loss than carbon particles.

Patented August 2, 1966 layer will not feed back to affect the photoconductive layer, and so the light emitted from the electroluminescent layer at each point can be modulated. Such a screen is unnecessary to prevent feedback if the photoconductive material does not respond to the wavelengths of radiation emitted by the electroluminescent phosphor.

Other advantages, features and objects of the invention will be apparent from the following specification, taken 1n connection with the accompanying drawing in which:

FIG. 1 is a schematic representation of one embodiment of my device;

FIG. 2 is a representation of another embodiment of my device; and

FIG. 3 is a schematic diagram of apparatus in which an image is amplified according to my invention.

In FIGURE 1, the electroluminescent layer 1 is applied over an electrically-conductive transparent coating 2 on a piece 3 of glass or other light-transmitting medium, and the layer 4, which can be a mosaic of separate particles or groups of particles or areas, of photoconductive material, is applied over the electroluminescent layer 1. A transparent electrically conductive layer 5 is over and in contact with the photoelectric mosaic layer 4, said electrically-conductive layer being on the surface of the plate 6 of glass or other light-transmitting material.

The electroluminescent layer 1 can be of phosphor particles embedded in dielectric material, as shown in copending application, Serial No. 180,783, Patent Number 2,838,715 filed August 22, 1950 by Elmer F. Payne or in application Serial No. 365,617, now abandoned, filed July 2, 1953, by Richard M. Rulon, or in some other convenient manner. The phot-oconducting layer 4 can be of selenium, cadmium sulphide or the like, materials now well-known in the art. The transparent conducting layer can be, for example, of stannous chloride or the like applied on glass or other material as shown in said Rulon application, or in other manners known in the art.

The phosphors can be of copper-activated zinc sulfide, as shown in said Payne application, or in copending applications Serial No. 230,711, Patent Number 2,772,242, filed June 8, 1951, and Serial No. 230,713, Patent Number 2,728,730, filed June 8, 1951, respectively by Keith H. Butler and by Keith H. Butler and Horace H. Homer, or can be any other suitable electroluminescent materials.

In FIG. 2, the device is as in FIG. 1 with the addition of a light-absorption layer 7 between the electroluminescent layer 1 and the photoconductive layer 4. This layer can contain particles of a dark pigment suspended in a dielectric material, for example, carbon black particles in one of the dielectric materials shown in the copending application previously referred to. High-resistivity black particles, for example, cupric oxide, would produce less A white paint of high reflectivity could also be used as the opaque or lightabsorbing layer, for example a coating of barium titanate, which is white and has a high dielectric constant, could be used.

The purpose of the opaque or reflecting layers is to prevent feedback from the electroluminescent layer 1 to the photoconductive layer 4. When the intensity of light emission does not need to vary over the face of the layer, the opaque layer can even be of a low resistance metal such as aluminum. In such a case, a high light output could be controlled by a low light input, but no image would be produced, because the voltage across the elec- 'troluminescent layer would be the same over its entire area.

If the opaque layer is applied as a mosaic like the photoconductive layer, and preferably with its particles or areas in register with and in contact with corresponding particles or areas of photoconductive material in layer 4, then the opaque layer can be of a low resistance material such as aluminum Without preventing variation in light emission from point to point over the electroluminescent layer, that is, without becoming. an equipotential surface.

The foregoing embodiments will be useful when the exciting voltage is alternating, and event when it isvarying direct current. When direct current is used, however,

it is preferable to omit the embedding dielectric material.

and make layer 1 wholly of electroluminescent phosphor I In FIG. 3, a small cathode ray tube 11 has an image produced on its screen 12 which is enlarged by the lens 13, focussed onto the photoconductive layer 4. The

opaque .layer 7 is in contact with-the side of layer 4- opposite to that on which the image is focused,'and an electroluminescent layer is in contact with the other side of the opaque layer 7. Transparent conductive layers 2, 5,, are in contact with the outside surfaces of layers 4 and 1, andare backed by the, glass supporting plates 3, 6. An external source'of voltage V is connected between the conductive layers 2, 5.

In operation, an image is produced on the screen 12 of cathode ray tube 11 in the usual manner, for example, a moving picture as produced when the tube 11 is the picture tube of a television set. The image is enlarged by the lens 13 and focused on the photoconductive layer 4, where it will appear in less bright form due to the in-.

crease in its area. The light fial'ling on the photoconductive layer 4 will reduce the resistance of that layer;

however, and that will increase the voltage across the electroluminescent layer, thereby causing the latter to emit light. This light can be much greater in intensity than the light received on the photoconductive layer 4.

I The resistance of the photoconductive layer 4 is de.- sirably made high enough so that the light emittedby the electroluminescent layer will .be negligible-When no light falls on the photoconductive layer, despite the presence of a voltage across the whole device in series. In many cases, it will be desirable to makethe photoconductive layer .4 of a material which will respond to ultraviolet radiation and be relatively insensitive to visible light,.so that, it will not be affected by the general illumination in the room. However, visible light or even infra-red can be used, with proper choice of the photoconductive ma-w terials, of which many are'known in the art;

The photoconductive layer 4 can be applied to the electroluminescent layer 1 by spraying, evaporating, setting, painting, or the like through a screen, for example,

through a sheet oi paper or of metal having holes therein, the holes being close together, for example, the

pe-rimeters of the holes could be spaced apart a distance equal to about one-fourth of the radius. For example, in a 25-inch screen the particles of area of the mosaic .can

be about 0.1 inch between centers for a 250-line screen,

4 andabout 0.6 .to 0.8 inch in;diameter if, the. areas are roundor in length if the areaszare square;

The exciting voltage can be alternating, afor example,

60 cycles persecondr It should be high enough tocause no observableflicker.

With respect to the above, .it. is important to .note that although the electron beam'in-the cathode ray'tube 11 1 may. fall on aparticular :spot of the cathode ray tube screen 12 for only. say, 5 of a second, the light emitted from that spot will continue for a considerably longer -time,.say 0.1 second, because the emissionwill' have an appreciable decay time or: phosphorescence. The image falling on photoconductive layer 4 will thus have a period determinedby that decay time and willnot be influenced by the time .that the cathode ray. beam in tube 11 is in register with a particular spot. In other Words, no pulses Of the second durationmentioned will appearin the radiation exciting screen .12.

This application is based in part upon my copendin'g application, Serial No. 439,018, Patent Number 2,818,531 filed June.24', 1954.

What I cla-im'is:

1. An electroluminescent device comprising an electroluminescent layer .with a layer of lightrreflecting di-' electric material thereovenzand a layer of photoconductive material over said dielectric layer.

2.1 .An electroluminescent unit comprising .an electroluminescent device and a photoconductive device in series, said electroluminescent device :including an electroluminescent phosphor embedded in a dielectric material.

References Cited by-the' Examiner UNITED STATES PATENTS OTHER i REFERENCES Bramley et 211., Physical Review, vol. 87, No.6, Sept. 15, 1952,.page1125.

Encyclopedic: Dictionary of Physics, vol. 4, 1961, page. a

plement 1, 1963,=Academic.Press.

Luyckx et :al., British Journal of; Applied Physics, Supplement No. 4, page, 57 '(1955).

Orthuber .et al.,i A Solid-State Image. Intensifier, J. Opt. Soc..Am., vol. 44, April 1954, pp.g2917j299.

RALPH G. NIL'SON; Primary Examiner.

ELI I. SAX, RICHARD'M. WOOD, MAX L. LEVY,

Examiners.

V. LAFRANCHL'W. STOLWEIN; Assistant Examiners.

Ivey, ,Electrolu'minescence. 33nd Related Effects, 7 Sup- 

1. AN ELECTROLUMINESCENT DEVICE COMPRISING AN ELECTROLUMINESCENT LAYER WITH A LAYER OF LIGHT-REFLECTING DIELECTRIC MATERIAL THEREOVER, AND A LAYER OF PHOTOCONDUCTIVE MATERIAL OVER SAID DIELECTRIC LAYER. 